The present invention is related to an amplifier circuit suitable for amplifying at least two different signals such as signals of two different wavelength bands.
In communication with mobile stations there nowadays is a need for using different standards which can require modulation methods suited for the respective amplification methods and hence different amplifier designs can be required. Also different wavelength bands are used in the different standards. In a dual band mobile station the station can receive and transmit signals in two different wavelength bands, e.g. using GSM at wavelengths at 1800 MHz and AMPS at 800 MHz.
In a mobile station such as a mobile telephone set a power amplifier is used in the final step of transmitting signals from the station. In the power amplifier power transistors are used for amplifying the signals. These transistors usually comprise FETs (Field Effect Transistors) of various designs. The output power required for different standards may be different for the respective wavelength bands and thus the output FETs must be driven to produce an output power which is different for each band. For instance, for 900 MHz an output power of 3.5 W can be required and for 1800 MHz the required output power can be 1.6 W. There exists a multitude of standards using different wavelength band such as AMPS for 800 MHz as mentioned above, GSM for 900 and 1800 MHz and PCS for 1900 MHz. Also, the output transistors may have to be designed differently for different standards owing to the use of different modulation methods which require specially selected operating points, etc.
FETs used as output amplifiers can only be designed to optimally produce an output power for a particular small range of the output power. In FIG. 1 a diagram of the efficiency xcex7 of a FET is plotted as a function of the output power for some special assumptions which can e.g. include constant drain-source voltage, constant load resistance, only the average gate-source being varied, etc. In the diagram FIG. 1 two graphs are plotted, one for the efficiency of FET optimized for the output power for transmitting at 900 MHz according to some standard and another one for transmitting at 1800 MHz according to some similar standard, which however requires a lower output power. The respective transistors can only be used in the regions lower than the respective shaded area. Suitable operating points are indicated by the dashed lines. It is observed in the figure that a FET designed to optimally work to provide the power required at 900 MHz will have a lower efficiency when used for providing a lower output power at a higher frequency such as 1800 MHz under these assumptions. Generally, thus it is concluded that it is difficult to design an optimized single output FET for a multimode power amplifier. In the example of FIG. 1 different FETs should used which are designed to optimally provide the required output power at the two frequencies. The two FETs could then e.g. each have as small an area as possible to be suited to provide the respective output power.
However, also the power consumption and the cooling of the FETs must be considered. There is generally an increasing demand for transmitting data from the mobile stations using standard networks. In such networks the time for communicating with a considered base station is divided in time slots so that transmission from a considered mobile station can be made say at most every eighth time slot. This means that if a FET is designed for a particular output power, the power consumption and the power dissipation will only correspond to about an eighth of the amounts required for a continuous transmission. When data transmission is required, more time slots can be assigned to the mobile station, e.g. two or three time slots out of a set of totally eight time slots. This in turn implies that the power amplifiers and the FETs comprised therein must be designed for this higher average output power and thus the cooling of the power circuits has to be improved. This would generally require that the area of the FETs had to be considerably increased in order to enhance the conduction of heat away from the transistors.
In U.S. Pat Nos. 4,276,516 and 4,682,197 interdigitated bipolar transistor structures for power amplifiers of Class B are disclosed in which neighbouring fingers comprise the complementary transistors which conventionally are alternatingly switched on in such amplifiers.
It is an object of the invention to provide an amplifier circuit for two different wavelength bands which has an efficient cooling and an efficient use of chip area.
The problem to be solved by the invention is thus how to integrate the different transistors needed for amplifying signals of two wavelength bands in order to occupy a small area on a semiconductor chip and to have transistors located so that the power dissipated by the transistors can efficiently be conducted away.
In a structure of power field effect transistors and for instance used in an output stage of signal processing circuit each individual transistor comprises a multitude of xe2x80x9cfingersxe2x80x9d, the fingers being element or elementary transistors located to form a line or row. By interdigitating the fingers so that element transistors belonging to one power transistor alternate with transistors belonging to another power transistor each element transistor will have a large surface area of a chip which will receive the heat developed by the element transistor. This presupposes that at each instant only one power transistor is active which is true for instance when communicating using different wavelength bands and each wavelength band has its own power transistor.
The element transistors are thus collected in groups to form field effect transistor units which are used for different purposes. One transistor unit can thus be intended for amplifying signals of wavelengths or of wavelength band requiring more electric output power such as a lower frequency band. Since this transistor unit has to supply an output signal having a large output power, it hence requires a large area of a chip. Another transistor unit can require a smaller area since when it is active less heat is developed, such a transistor unit for instance being intended to amplify signals of a higher frequency. The number of element transistors of a first transistor unit can thus be larger than the number of element transistors of a second transistor unit. Alternatively or combined therewith the element transistors of the second transistor unit can each one occupy a smaller area than the element transistors of the second transistor unit.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.